Field of the Invention
This invention relates to equipment and methods for performing surgery, more specifically the invention relates to a customized fixation and targeting system for stereotactic frames.
Description of Related Art
In many medical procedures, it is of utmost necessity that the patient be stabilized in a fixed, unmoving position. This is particularly true of surgery that occurs in the region of the head, neck, and spinal cord. The precise measurements and movements that are involved with surgery of this type demand that the patient be immobilized so that a surgeon's movements can be carefully calibrated to a known safe trajectory through critical regions of, for example, the cerebral cortex or the spinal column.
An additional consideration for these surgeries, as alluded to above, is that a safe, known trajectory must exist in the situations where a surgeon must reach deep tissue, for example, the subthalamic nucleus (STN). Given the location of this brain structure, a surgeon must plan a careful route for accessing the region that will not compromise other areas of the brain that lie between the skull and the deep brain, such as motor and sensory regions.
As the need for precise surgeries has become more prevalent, novel means of planning a safe route to deep tissue and of ensuring that the route remains static during surgery have been developed. One of the most common is the stereotactic frame, which has been in use in one form or another since the 1940s. For example, U.S. Pat. No. 4,341,220 discloses a frame fixed to a patient's head and defining a three-dimensional coordinate system in which a surgical device may be precisely positioned. A frame such as disclosed therein may also have markers detectable via X-ray, so that when the frame is attached to the patient's skull, markers on the frame are visible on a scan, for example a computed tomography (CT) scan, along with the brain structure of interest.
The process of scanning a patient with a stereotactic frame containing visible markers is referred to as registering the frame to the patient. The way this registration is accomplished is to scan the patient with the stereotactic device attached. The device then acts as a known reference point in relation to the patient's anatomy. A surgeon may then visualize a route to the target by utilizing the coordinates of the markers and of the brain structure, allowing for a pre-planned route that avoids other critical areas. In this way, the stereotactic frame can be manipulated by the surgeon to provide a customized path to the target structure. These procedures, however, are not without substantial drawbacks.
One common modern device for these surgeries is a frame such as disclosed in U.S. Pat. No. 5,423,832, which includes a head ring attached to the patient by four fixation posts holding pins that screw in to contact the skull. These pins, such as those disclosed in U.S. Pat. No. 5,300,076, must be secure enough to hold the weight of the stereotactic frame and in some cases the weight of the patient's head in a prone position without slipping. To do this, the pins must penetrate skin and bone, creating a divot in the skull. This can be a painful and disturbing process for the patient. Using the adjustments available in the fixation posts and the pins, the surgeon attempts to align the head ring with known but not visible anatomical points in the brain that will be visible in the scan, and with planes that coincide with anatomical atlases used to navigate the brain. This is difficult under the circumstances, and the alignment is never perfect.
In a typical modern cranial surgical procedure, after the ring is attached, a fiducial box with radiopaque markers or bars of known geometry is fixed to the ring. Once the patient has been scanned with the head ring and fiducial box attached, the images are loaded into a computerized surgical planning computer. Here the fiducial markers are plotted and the location and orientation of the stereotactic frame coordinate system (FCS) based on the head ring is determined in relation to the patient's anatomy in the images. A surgical target is located in the patient images, and an entry point on the skull is determined. Based on these locations, frame adjustment settings are determined that will provide a trajectory from the frame instrument mounting location to the target through the selected entry point. These calculated settings correct for the misalignment of the frame to the patient's anatomical reference system (PRS) and then find an adjustment setting that puts the frame's FCS center at the selected target. This surgical planning may take some time, since the surgeon must determine a trajectory that avoids blood vessels and areas that could be damaged by instrument penetration, causing complications, injury, or death.
While this surgical path is planned, the patient is in an awake condition, often experiencing discomfort and anxiety. The ring cannot be removed without losing the registration that has been achieved through the previous scanning. The operating room is prepared for the surgery, but since the planning has to be done after the patient is scanned, and the period of time required varies, it is difficult to schedule the surgical procedure exactly, leading to wasted valuable operating room time. After the planning is done the patient is taken to the operating room, and the adjustable parts of the stereotactic frame are set to the calculated settings to reach the target and then attached to the head ring.
Some stereotactic frame systems have what is called a “phantom”, which is used before attaching the frame to the patient to verify the correct settings of the frame. The identical settings are dialed into the phantom and its duplicate head ring. The upper frame portion is then attached to the duplicate head ring and a probe is used in the adjustable frame portion to determine that it places the probe tip at the correct target center. The phantom does not verify targeting, since it has no reference to patient anatomy. It verifies only that the frame is set correctly to the calculated figures.
In addition to providing precise paths through brain tissue, head rings and stereotactic frames described above can also be used for securing a patient during sensitive irradiation procedures. The above-described method of registering is similar. However, rather than attach a stereotactic frame with an adjustable guide for surgical procedures, the ring itself is connected to an irradiation device, immobilizing the patient in a position calculated based on the known relationship between the head ring and the patient's brain structures so a concentrated focal point of radiation targets, for example, a tumor or lesion, is in a known position relative to the trajectory of irradiation beams. One example of such a device is the Elekta Gamma Knife®. Practically, however, this current method limits the patient to one or a very few number of procedures, since repeated treatments require a reapplication of the head ring, rescanning, and re-planning.
In newer versions of such devices, the travel and size of the treatment area have been increased to allow the radiosurgical treatment of tumors and lesions in the neck and cervical spine. The use of this new device for such treatment has been limited due to the difficulty of fixating the head and neck in relation to the upper body of the patient. Since the neck is capable of a great range of motion, the use of the current head ring mounting system of the standard stereotactic frame does not sufficiently restrain the patient's neck and cervical spine enough to allow safe and accurate targeting of neck and upper spine lesions.
The drawbacks to the existing technology are numerous. Current head rings rely on fixation posts that penetrate skin and bone and exert pressure on the skull of a patient, resulting in a painful and anxious experience. Initial placement and alignment of the ring is difficult due to the surgeon's inability to see the underlying brain structures, and the relation of the ring to those structures, until after the initial scan. Once the ring is attached, it cannot be removed, for example in a situation where multiple surgeries or treatments are required, without the need to re-register the ring to the patient, followed by time-consuming re-planning of the surgical or treatment route. Once the frame is attached to the ring, alignment is often inconsistent with the PRS and relies on an algorithm to correct any misalignment. Additionally, as new techniques become available for procedures on the neck and spine, the current frame technology is incapable of safely securing patients for precise treatment of those areas.
Accordingly, there is a need in the art for a fixation and targeting system that is less invasive for the patient, but that provides a high level of precision and accuracy for the surgeon engaged in procedures of the head, neck, and spine.